The present invention relates to a combustor for a gas turbine system utilized for a compound cycle power plant, and more particularly, to a combustor for a catalytic combustion type gas turbine system.
In these days, in a view point of an effective utilization of an energy source, various types of gas-turbine/steam-turbine compound cycle power plants have been adapted. Such power plants, however, involve significant problems or objects of reducing nitrogen oxide (NOx) discharged from the power plants particularly in the view point of an environmental protection.
Such a conventional compound cycle power plant includes a homogeneous type combustion system in which an air/fuel mixture is ignited by a spark plug means, for example. However, in a combustor for such a conventional gas turbine, a highly heated (high temperature of more than 2000.degree. C.) portion is locally caused in the combustor at the time of fuel burning, nitrogen (N.sub.2) decomposed from air in atmosphere is combined with oxygen (O.sub.2), and thus, a large amount of NOx is generated, also providing a significant problem.
In the prior art, there is further studied and developed various combustion systems for solving such problems, and in recent years, there is provided a catalytic combustion system utilizing a solid phase catalyst.
FIG. 9 is a schematic diagram representing one example of such a conventional combustor utilizing the catalytic combustion system. Referring to FIG. 9, the combustor includes a cylindrical casing 1 and a combustion cylinder (heating cylinder) 2 coaxially arranged inside the casing 1. The combustion cylinder 2 has one end wall, lefthand end as viewed, separated from one end wall of the casing to define a gap therebetween. An annular circumferential space is also defined between the inner combustion cylinder 2 and the outer casing 1, and the gap and the space are communicated with each other so as to form a combustion air supply passage 3.
The end wall, i.e. lefthand end wall, of the combustion cylinder 2 has a central portion at which a nozzle 4 for a diffuse fuel (diffuse fuel nozzle 4) is disposed and a swirler 5 is arranged around the outer peripheral portion of the diffuse fuel nozzle 4. A catalyst unit 6 in which a catalyst for the combustion of honeycomb structure utilizing a solid phase catalyst is located in the combustion cylinder 2, and a mixture gas supply port 8, through which a mixture gas of a fuel for the catalytic combustion (catalytic combustion fuel) supplied through a nozzle 7 for the catalyst (catalyst fuel nozzle 7) and an air for combustion is supplied, is formed on an upstream side, facing the diffuse fuel nozzle 4 in the illustration, of the catalyst unit 6.
On the downstream side of the catalyst unit 6 in the combustion cylinder 2, there is formed a nozzle 9 for a premixture fuel (premixture fuel nozzle 9), and an enlarged zone or portion 10 is also formed further on the downstream side of the premixture fuel nozzle 9 for uniformly mixing the mixture gas. A rear, righthand end as viewed in FIG. 9, of the enlarged portion 10 is connected to a turbine nozzle, not shown. The mixture gas is burned by means of ignition plugs 11 and 12 having ignition points positioned in the mixture gas supply port 8 and the enlarged portion 10 in the combustion cylinder.
The fuel is classified into or composed of a fuel for diffuse combustion (diffuse combustion fuel) F1, a fuel for calalytic combustion (catalytic combustion fuel) F2 and a fuel for premixture combustion (premixture combustion fuel) F3, which are supplied into the combustion cylinder 2 respectively through the diffuse fuel nozzle 4, the catalyst fuel nozzle 7 and the premixture fuel nozzle 9. On the other hand, air for combustion (combustion air) is supplied into the combustion air supply passage 3 through a front end opening of the passage 3 as shown by an arrow in FIG. 9, and then supplied into the combustion cylinder 2 in part through the swirler 5 and in remaining part together with the catalytic combustion fuel F2 through the supply port 8. The combustion gas subsequently burned in the cylinder 2 is jetted into a gas turbine through the turbine nozzle connected to the enlarged portion 10 at the rear end of the combustion cylinder 2.
FIG. 10 is a graph showing a relationship between a fuel flow rate and a combustor load in connection with a fuel rate control, in which the abscissa axis represents a combustor load and the ordinate axis represents a fuel flow rate.
With reference to FIG. 10, an ignition is carried out at the point Pa, and the diffuse combustion fuel F1 from the diffuse fuel nozzle 4 and the premixture combustion fuel F3 from the premixture fuel nozzle 9 are supplied into the combustion cylinder 2 up to the time when the combustor load reaches the point Pe. When the combustor load exceeds the point Pe, a condition for causing the catalytic combustion is realized (for example, a temperature at the inlet portion of the catalyst unit reaches a predetermined temperature), so that the catalytic combustion fuel F2 is supplied into the combustion cylinder 2 through the catalyst fuel nozzle 7 to then carry out the combustion in the catalyst unit 6 and to adjust the flow rate of the diffuse combustion fuel F1 so as to maintain an optimum gas temperature at an inlet of the catalyst unit 6. A curve B in FIG. 10 represents a change of a total fuel flow rate. As shown in FIG. 10, at the combustor load more than point Pe, almost half the fuel can be burned through the catalytic combustion, thus suppressing the generation of NOx.
FIG. 11 represents a temperature change of the gas temperature at the inlet portion of the catalyst unit 6 with respect to the combustor load.
However, the above-mentioned catalytic combustion system involves such problem as requires an increased temperature more than a predetermined one for starting the catalytic combustion. It is required at present for the catalytic combustion starting temperature to be higher than a temperature of the combustion air to be supplied with respect to almost catalysts, of course, being different in accordance with kinds or types of catalysts and fuels to be utilized. Accordingly, it is necessary to increase the temperature of the air/fuel mixture gas to be supplied to the catalyst unit up to the catalytic combustion starting temperature. For this purpose, the diffuse combustion is to be performed with respect to a part of the fuel.
In such method, the diffuse combustion provides a stable combustion, but has a high burning temperature, which results in generation of the NOx. The NOx generated during this diffuse combustion process constitutes almost part of the NOx generated in the entire combustor. Accordingly, this method provides a NOx suppression effect more than that of the conventional combustor utilizing no catalytic combustion system. However, as stated above, this method is not applicable for a present or future combustor to which more severe NOx prescription will be required.